Telephone service within buildings is provided by cables some of which are referred to as rise cables. These cables generally used to interconnect between equipment closets located on various floors or to extend from vaults in basements to other floors. Risers are employed mainly for vertival runs between floors within buildings. They may extend between service entry locations, equipment closets, and equipment to other building floors where they are connected to plenum cables distribution cables, distribution equipment centers or directly to equipment such as computers, telephone or facsmile apparatus, for example. Typically, a riser cable includes a core having a plurality of optical fibers or a plurality of twisted pairs of metallic conductors which are individually enclosed with an insulation cover. the core is enclosed in a sheath.
Because of the environment in which these riser cables are used, they must meet specified requirements which relate to fire-retardancy and mechanical integrity. Besides plenum cable, there are four categories of flame test requirements for cables which may be used in building applications. First there is the riser class and a cable qualifying for same must meet the requirements of Underwriters Laboratories, Inc. (UL) 1666 flame test. Another standard is that of the Canadian Standards Association which require that any candidate cable meets the requirement of its FT-4 test. There is a general use class in which the candidate cable should meet the requirements of a general purpose test embodied in UL 1581 vertical tray flame test. A fourth category is referred to as VW-1 which defines a standard for a vertical transmission medium for residential use. These flame test requirements are less stringent than those for plenum cable.
Another consideration is the pair count density, which is the number of insulated conductors in a given cable cross-section. With the trend toward larger and larger buildings and the increased use of the telephone for various kinds of communication, the pair count density within a building riser system may be greater than that in the past.
Also of importance to building cables is the capability of color coding the conductor insulation. Typically, a predetermined number of conductor pairs are grouped together in what is referred to as a unit. The unit is characterized by unique color combinations among the pairs as well as a binder having a particular color. This allows an installer to be able to identify a particular conductor pair and to distinguish between tip and ring. As a result of the relative ease of identification, splicing and termination costs are greatly reduced.
The prior art has addressed the problem of cable materials that contribute to flame spread and smoke evolution through the use of halogenated materials such as fluoropolymers and polyvinyl chloride (PVC). For example, these together with layers of other materials, have been used to control char development, jacket integrity and air permeability to minimize restrictions on choices of materials for insulation within the core in a plenum cable. Commercially available flourine-containing polymer materials have been accepted as the primary insulative covering for conductors and as a jacketing material for plenum cable without the use of metal conduit. In one prior art small size plenum cable, disclosed in U.S. Pat. No. 4,605,818 which issued on Aug. 12, 1986 in the name of C. J. Arroyo, et al., a sheath system includes a layer of a woven material which is impregnated with a fluorocarbon resin and which encloses a core. The woven layer has an air permeability which is sufficiently low to minimize gaseous flow through the woven layer and to delay heat transfer to the core. An outer jacket of an extrudable fluoropolymer material encloses the layer of woven material. In the last-described cable design, a substantial quantity of flourine , which is a halogen, is used.
Besides flouropolymers, other halogenated material such as chlorine-containing polymers e.g. PVC, are also used for both insulation and jacket. One such cable is disclosed in U.S. Pat. No. 4,412,094 which issued on Oct. 25, 1983 in the names T. S. Dougherty, et al. In it, a riser cable is disclosed to have conductors insulated with a composite insulation comprising an inner layer of an expanded polyethylene and an outer layer of PVC. A PVC jacket encloses the core.
Although the PVC is very attractive in terms of cost, when it is compared with other polymers, namely, flourinated polymers, it has relatively high dielectric properties which make it insuitable for high frequency applications. In addition, some PVC materials exhibit a relatively high degree of corrosivity and smoke generation in fire situations although others which are highly filled with halogenated materials are acceptable for plenum use.
The problem of acceptable building cable design is complicated somewhat by a trend to the extension of the use of optical fiber transmission media from a loop to building distribution systems. Not only must the optical fiber be protected from transmission degradation, but also it has properties which differ significantly from those of copper conductors and hence requires special treatment. Light transmitting optical fibers are mechanically fragile, exhibiting low strain fracture under tensile loading and degraded light transmission when bent with a relatively low radius of curvature. The degradation in transmission which results from bending is known as microbending loss. This loss can occur because of coupling between the jacket and the core. Coupling may result because of shrinkage during cooling of the jacket and because of differential thermal contractions when the thermal properties of the jacket material differ significantly from those of the enclosed optical fibers.
The use of fluoropolymers, with or without underlying protective layers, for optical fiber building cable jackets requires special consideration of material properties such as crystallinity, and coupling between the jacket and an optical fiber core which can have detrimental effects on the optical fibers. If the jacket is coupled to the optical fiber core, the shrinkage of fluoropolymer plastic material, which is semi-crystalline, following extrusion puts the optical fiber in compression and results in microbending losses in the fiber. Further, its thermal expansion coefficients relative to glass are large, thereby comprising the stability of optical performance over varying thermal operating conditions. Also, the use of some fluoropolymers requires special care for processing and adds to cost of the cables.
Although there exists cables which include halogen materials and which have passed the UL test requirements, there has been a desire to overcome some problems which still exist with respect to the use of some fluoropolymer and PVC halogenated materials. Both these materials may exhibit undesired levels of corrosion. If a fluoropolymer is used, hydrogen flourife forms under the influence of heat, causing corrosion. For a PVC, hydrogen chloride is formed. Futher, some PVC materials exhibit an undesired level of smoke generation.
Generally, there are a number of halogenated materials which pass the industry tests for building cable. However, if some halogenated materials have undesirable characteristics and industry demands certain characteristics that halogenated materials are lacking, it is logical to inquire as to why non-halogenated materials have not been more widely used for building cable materials. the prior art has treated non-halogenated materials as unacceptable because as a genral rule it has been said that they are not as flame retardant or because they are too inflexible if they are flame retardant. Materials for use in communications cables for building applications must be such that the resulting cable passes an industry standard flame test, which for riser cables, is the hereinbefore mentioned UL 1666 test. In addition, the cable should pass physical and mechanical test set forth by UL as well as transmission requirements set forth by manufacturers such as AT&T.
Non-halogenated materials have been used in countries outside the United States particularly in the less stringent categories for building cable. One example of a non-halogenated material that has been offered as a material for insulating conductors is a polyphenylene oxide plastic material. Ongoing efforts have been in motion in the United States to provide non-halogenated material which has a broad range of acceptable properties, as well as a reasonable price, and yet one which passes industry standards such as the UL 1666 test for riser cables. Such a cable should be one which appeals to a broad spectrum of customers.
One such cable for plenum applications is that disclosed in copending commonly assigned application Ser. No. 07/303,212 which was filed on Jan. 27, 1989 in the names of T. G. Hardin and B. A. Khorramian. In it is disclosed a cable in which transmission media are enclosed in a covering material which is selected from the group consisting of a polyetherimide, a polymide, a silicone-polymide copolymer and blend compositions of a polyethermide and a silicone-polymide copolymer. A jacket which encloses the transmission material is a plastic material which includes a polythermide constituent. The cable of the aforementioned application meets UL 910 test requirements for plenum cable exhibits relatively low corrosivity and a suitable toxicity level. However, the materials which are used for insulation and jacketing require somewhat more care to process than conventional cable materials such as polyethylene and PVC. Also, such a cable, although suitable for riser cable, far exceeds the requirements for same. In addition, such a plenum cable costs considerably more than what is perceived to be reasonable for riser cables, a situation which is well received by the customer.
The sought-after riser cables not only exhibit suitably low flame spread producing characteristics provided by a currently used cables which include halogenated materials but also one which meets a broad range of desired properties such as low smoke generation, acceptable levels of corrosivity and toxicity and which is reasonable in cost. Such a riser cable does not appear to be available in the prior art. The challenge is to provide such a cable which meets the standards in the United States for riser cables.
what is needed is an insulation and jacketing system for a riser cable which minimizes the opportunity for the beginning of a fire alon the cable, and should such a flame be initiated, one which minimizes the propagation of the flame and the total heat which is released by the cable system. Also, the sought-after insulation not only must have a relatively small diameter-over-dielectric in order to reduce the outside diameter of the cable, but must also must lend itself to a color coding scheme in order to facilitate inside wiring and splicing.
What is needed and what does not appear to be available is a building cable which meets the requirements of Ul tests for riser applications. The sought-after cable desirably is one which uses non-halogenated insulating and jacketing materials, yet in one which satisfies the UL requirements for use in building risers at a reasonable cost with excellent transmission characteristics.